Converts MongoDB Atlas connection url (or cluster name) into regular mongo:// URL with initial host randomization.

mongo-url.js

const dns = require("dns");

/**
 * Converts MongoDB Atlas connection url (or cluster name) into regular URL,
 * with initial host randomization.
 *
 * Uses so-called DNS Seed List Connection Format
 * @see https://www.mongodb.com/docs/manual/reference/connection-string/#dns-seed-list-connection-format
 **/
const convertMongoAtlasConnectionUrl = (url) =>
  new Promise((resolve, reject) => {
    let host = url || process.env.MONGO_URL;
    let db = process.env.MONGO_DB || "db";
    let query = "";

    if (!host) {
      reject(new Error("No url or hostname specified!"));
    }

    try {
      const url = new URL(host);
      host = url.host;
      query = url.search ? url.search.substring(1) : "";
      db = url.pathname.length > 1 ? url.pathname.substring(1) : db;
    } catch (e) {
      // Don't care - consume either url or hostname
      console.info(e);
    }

    const srv = "_mongodb._tcp." + host;
    dns.resolve(srv, "SRV", (err, addresses) => {
      if (err) {
        const e = new Error("Failed to resolve SRV " + srv + ": " + (err?.message || String(err)));
        e.error = err;
        reject(e);
      } else {
        const nodes = addresses.map((a) => a.name);
        dns.resolve(host, "TXT", (err, values) => {
          if (err) {
            const e = new Error("Failed to resolve TXT " + host + ": " + (err?.message || String(err)));
            e.error = err;
            reject(e);
          } else {
            let options = values.flat(1).join("");

            if (options.indexOf("connectTimeoutMS=") < 0) {
              options += "&connectTimeoutMS=" + (process.env.MONGO_CONNECT_TIMEOUT || "3000");
            }

            if (options.indexOf("ssl=") < 0 && options.indexOf("tls=") < 0) {
              options += "&ssl=true";
            }

            if (query) {
              options += "&" + query;
            }

            const random = nodes[Math.floor(Math.random() * nodes.length)];
            resolve("mongodb://" + random + ":27017/" + db + "?" + options);
          }
        });
      }
    });
  });

if (require.main === module) {
  convertMongoAtlasConnectionUrl(process.argv[2])
    .then((url) => process.stdout.write(url))
    .catch((e) => {
      console.warn(e?.message || String(e));
      process.exit(1);
    });
}